Metal detector



Oct. 30, 1962 J. H. 'REZNOWSKI METAL DETECTOR Filed May 4. 1959 '"AIII'IIIII'IIIIIIm/II 40 FIG.4

VENTOR. Z N 0W5 KI IN JULIAN H. RE

OSCILLATOR BY BUCKHORN CHEATHAM & BLORE ATTORNEYS with a variable resistor.

'United States i atent 3,061,775 METAL DETECTOR Julian H. Reznowsiti, Chehalis, Wash, assignor to Industrial Electronics Incorporated, Chehaiis, Wash, a corporation of Washington Filed May 4, 1959, Ser. No. 810,908 1 Claim. (Cl. 32441) The present invention relates to a metal detector and more particularly to apparatus for detecting the presence of metal in wood chips or other non-metallic materials carried on a conveyor.

The scrap materials in a lumber operation, that is slabs, chips and the like, are frequently fed to a fuel hog which reduces the scrap material to small size so that it can be fed to wood burners and used to generate power for the operation. Frequently, also, Wood is fed to chipping apparatus for making chips for chipboard and for other purposes. Obviously, if metal objects are in such wood the cutting knives doing the chipping may be damaged and injury might be caused to personnel working in the vicinity.

It is an object of the present invention to provide a new and improved detector for detecting the presence of metal objects in a mass of Wood or other non-metallic material on a moving conveyor so that the metal can be removed.

Still another object of the invention is to provide a metal detecting unit that is relatively insensitive to variations in the moisture content of Wood chips or other material being assayed.

A further object of the invention is to provide new and improved means for securing a minimum signal in the detecting unit in the absence of such metal objects.

Still other objects and advantages of the invention will become more apparent hereinafter.

in accordance with the illustrated embodiment, the present invention comprises a detector unit including an energizing coil supplied with alternating current so as to provide an alternating magnetic field and a detector coil arranged so as to provide substantially minimum coupling between such coils. The detector coil is connected to a suitable detecting and amplifying circuit and to signaling devices so as to indicate the presence of tramp metal in the material being conveyed by reason of voltages induced in the detector coil due to distortion of such field by the metal to thereby increase the coupling between such coils. T o facilitate balancing of the detector and energizing coil circuits to produce a minimum voltage input to the amplifier in the absence of such metal, I have provided a small balancing coil which is closely coupled to one of the other coils and connected in series Such coil forms a field distorting element which in conjunction with a suitable circuit enables such balancing by merely adjusting the variable resistor.

A capacitive shield in the form of a grounded grid is provided between the coils and the conveyor on which the conveyed material is carried, which shield minimizes the effect of varying moisture content within such material upon the sensitivity of the detector unit.

For a more detailed description of the invention, reference is made to the accompanying drawings wherein:

FIG. 1 is a diagrammatic view in side elevation of a conveyor for carrying scrap wood showing a metal detector arranged therewith;

FIG. 2 is a sectional view through a unit taken substantially along line 2-2 of FIG. 1;

FIG. 3 is a sectional view taken substantially along line 3-3 of FIG. 2;

FIG. 4 is a sectional view taken substantially along line 4-4 of FIG. 1; and

FIG. 5 is a circuit diagram of the electrical components of the apparatus of the present invention.

As has been stated previously, the apparatus of the invention is particularly useful in detecting the presence of metal objects in a mass of wood chips or scrap veneer and the like as such material is carried along a conveyor. Accordingly, the apparatus of the invention is shown diagrammatically in FIG. 1 in connection with a conveyor belt 10 carrying wood chips or wood scraps 12 thereon. The apparatus of the invention includes a detector unit 14 which is preferably mounted immediately beneath the conveyor and which unit is connected to a suitable amplifying and detecting circuit which may be contained in a housing 16 positioned at some convenient place remote from the detector unit 14 and which circuit may operate a visual or audio signal device 18 to indicate the presence of metal upon the conveyor and which may also be connected to control the motor 20 driving the conveyor 10 so that the conveyor may be stopped when the presence of metal is detected.

Referring now more particularly to FIGS. 2 to 5, inclusive, the detector unit comprises a flat detector coil 24 which is mounted in substantially coplanar relation with the conveyor 10 and a flat energizing coil 26 ar ranged with its plane perpendicular to the plane of the detector coil and with the energizing coil preferably extending transversely of the conveyor 10. The axes of the coils 24 and 26 are perpendicular to one another and the coils are preferably arranged so as to provide minimum coupling therebetween. The coils 24, 26 preferably are mounted within a suitable housing 28 so as to protect the coils against weather and from injury. Mounted adjacent to one of the coils 24 or 26 is a balancing coil 30. In the illustrated embodiment, the balancing coil 30 is mounted slightly out of parallel relation with the energizing coil 26 and with its axis eccentric to such coil. It is apparent that the coil 30 may occupy any other position in the alternating field of the coil 26 which will cause adjustable distortion of such field in a manner varying the coupling between the coils 24 and 26 when the coil is adjustably loaded. Suitable leads indicated at 32 are provided from each of the coils to a connector 34 in a Wall of the housing 28 into which a connecting cable 36 from the amplifier 16 may be plugged.

Positioned between the coils and the conveyor 10 is a grid structure which provides a capacitive shield between the detecting unit and the material being conveyed so as to minimize the effect of varying moisture content within the wood chips 12 upon the detector unit. In the illus trated embodiment, such a grid comprises a continuous wire 40 roved back and forth on the under surface of the top member 42 of the housing 28 and which grid is suitably grounded as indicated at 44 in FIG. 5.

Referring now more particularly to FIG. 5, the energizing coil 26 is energized from a suitable oscillator circuit indicated generally at 46 capable of operating at a frequency in the range between 2,000 to 10,000 cycles per second. Such oscillating circuits are well known to those skilled in the art and the coil 26 may, for example, provide the inductance of the tank circuit of a Colpitts type oscillator, which tank circuit also includes the capacitors 31 and 33.

The coils 24 and 26 are connected through a bridge circuit and transformer 58 to an amplifying circuit indicated generally at 60. The coil 30 has a variable resistor 62 connected across its terminals. The bridge circuit includes resistors R R R and R and capacitors C and C It will be apparent that the resistors R R and the capacitors C C can be considered to be a first bridge. A portion of the voltage across the energizing coil 26, namely that across the capacitor 33 is reduced by the voltage divider made up of the resistors 63 and 64 and 3 supplied through the series resistor 65 across one pair of opposed junctions of such first bridge. If such first bridge is balanced by making the ratio of R to R equal the ratio of C to C no voltage from the coil 24 will appear across the other two opposed junctions of such first bridge, i.e., across the detector coil 24.

Similarly, the resistors R R and capacitors C C can be considered to be a second bridge; A portion of the voltage across the coil 26 is applied to one pair of opposed terminals of such second bridge through the voltage divider including resistors 63 and 64 and through the series capacitor 66. Again it will be apparent that the second bridge will be balanced if the ratio of R to R is made equal to the ratio of C to C so that none of the voltage across coil 26 appears across the coil 24.

It will be further apparent that either or both bridges described above can be unbalanced by changing the values of resistors therein so that a portion of the voltage across the coil 26 does appear across the terminals of the coil 24. The dual bridges supplied with voltage from the coil 26 through the resistor 65 and capacitor 66 constitute also a phase shifting circuit so that such voltage may be made to have substantially any amplitude and phase within wide limits by selecting suitable values for the resistors R R R and R The detector coil 24 will not, in general, have zero coupling with the coil 26 so that a voltage is induced in such detector coil. If the coil 30 has the terminals open it 'will have little effect on the coupling between the coils 24 and 26 but loading such coil with the variable resistor 62 will increase the coupling between the coils 24 and 26 by distorting the alternating field produced by the energization of coil 26. In general, the lower the resistance of resistor 62, the greater the coupling between the coils 2 4 and 26 and the greater the voltage induced in coil 24. By selecting values of resistance for the resistors R R R and R producing a voltage of the same amplitude and 180 out of phase with the voltage induced in the coil 24 by mutual coupling with the coil 26, the current through the primary of the transformer 58 can be brought to a minimum value when no metal is present in the material being conveyed. Any metal in such material will further distort the field produced by the coil 26 to induce different voltage in the coil 24 so that the current through the primary of the transformer 58 increases to operate the detector or control devices through the amplifier 60. I V I In actual practice the values of the resistors R R R and R are selected at the point of manufacture of the metal detector to produce a minimum current in the primary winding of the transformer 58 when the variable resistor has its movable contact in an intermediate position. Any changes in surrounding conditions caused by installation of the metal detector at some other location which affect the coupling between the two coils 24 and 26 may then be compensated for by merely adjusting the variable resistor to produce minimum current in the primary winding of the transformer 58 as determined by any suitableindicating instrument. 7

Details of the amplifying and detecting circuit 60 have not been shown since any conventional circuit may be utilized so that when a piece of metal on the conveyor 10 comes within the field of the detector coil 24, the

resulting increased current flow through the primary winding of the transformer 58 will act through the amplifying circuit 60 to cause the alarm indicated at 18 to be actuated and to also actuate a relay 67 or other suitable control device to stop the motor 20 and halt the conveyor 10 so that the detected metal may be removed.

In the absence of the grid 40, moisture in wood being conveyed produces spurious response in the system described above at alternating current frequencies much above 2000 cycles per second. Apparently this is a capacitive etfect as the grid 40, constituting a capacitive shield, eliminates the efiect of varying moisture content in the wood chips 12 at considerably higher frequencies so that the detector does notv produce spurious signals at such frequencies. It has been found that the frequency of the detector oscillator can be increased from 2,000 to about 6,000 cycles ,per second and which, of course, increases the detecting efiiciency of the unit.

While I have described my invention in particular relation to detection of wood chips moving on a conveyor, it will be obvious that the detector unit can be utilized in other environments. Thus, having illustrated and described a preferred embodiment of the invention, it should be apparent to those skilled in the art that the invention permits of modification in arrangement and detail. I claim as my invention all such modifications as come within the true spirit and scope of the appended claim.

I claim:

Apparatus for detecting metal objects in a mass of wood chips on a moving conveyor comprising a detector unit mounted beneath said conveyor and including a detector coil disposed in coplanar relation with said conveyor, an oscillator coil mounted with its axis perpendicular to said detector coil and arranged to provide minimum coupling therebetween, a balancing coil mounted adjacent to one of the aforesaid coils, means for providing an alternating current of a frequency in the range between 2,000 and 10,000 cycles per second to said oscillator coil, means for detecting the induced voltage in said detector coil, a variable resistance connected in series to said balancing coil for producing a balance between said detector and oscillator coils in the absence of a metal object on said conveyor, a grid structure disposed between said coils and said conveyor, and means connecting said grid to ground whereby said grid structure provides a capacitive shield for said detector unit and minimizes the effect of varying moisture content within said wood chips upon the sensitivity of said detector unit.

References Cited in the file of this patent UNITED STATES PATENTS 2,237,254 Broekhuysen Apr. 1, 1941 2,463,252 Doll Mar. 1, 1949 2,547,407 Nelson Apr. 3, 1951 2,554,575 Kurtz et al. May 29, ll 2,598,252 Gossick May 27, 1952 2,623,923 Zimmerman Dec. 30, 1952 FOREIGN PATENTS 729,706 Great Britain May 11, 1955 

